The present invention generally pertains to broadcast signal receiving systems and is particularly directed to a system for automatically adjusting an apparatus for receiving a broadcast communication signal while the broadcast signal is being received. Such receiving apparatus include apparatus that may be adjusted to effect reception of a broadcast communication signal, such as an antenna, an antenna polarizer and a frequency-tunable receiver.
Typically, an antenna is attached to an antenna mount by an actuator and is rotated about a given axis on the antenna mount by moving the actuator in order to align the antenna with a given signal source, such as a communication satellite in geosynchronous orbit above the Earth's equator.
Once the antenna is aligned with the given signal source, it may also be necessary to align the polarizer of the antenna so that a linear polarization axis of the antenna matches a linear polarization axis of a communication channel being received from the signal source. Alignment of the polarizer is required when the signal source is a communication satellite, since odd and even numbered channels received from communication satellites are skewed ninety degrees with respect to each other in order to avoid interference between adjacent channels. One type of polarizer is aligned by rotating a feed probe within the polarizer.
The antenna and the polarizer are respectively realigned from their present positions in response to control signals provided by an antenna controller.
A typical frequency-tunable receiver is tuned to provide the received signal at a given intermediate frequency setting by adjusting the frequency of a local oscillator signal in response to operation of a control device to select a broadcast channel so that when the local oscillator signal is mixed with a received broadcast communication signal that is at the carrier frequency of the selected channel, the received signal is provided at the given intermediate frequency.
When one desires to change the received communication channel, the receiver is tuned to the carrier frequency of a newly selected broadcast channel, and an identification of the newly selected channel is provided to the antenna controller, which automatically effects a coarse realignment of the antenna and/or the polarizer, when the new channel is received from a different signal source and/or has a different linear polarization axis than the last previously received broadcast signal. Such realignment is effected in accordance with information stored in a data memory of the controller indicating certain alignment parameters of each of a plurality of channels.
In some prior art antenna controllers, fine alignment of the antenna and the polarizer are automatically achieved once the communication signal is being received following coarse realignment. Such fine alignment is achieved by measuring the amplitude of the received signal and realigning the antenna and/or the polarizer from an initial position until the measured signal amplitude is maximized. However, when interference and/or noise also are received over the communication channel, optimum signal reception is not always achieved at the maximum measured signal amplitude.
In some receivers, fine tuning to a given intermediate frequency is automatically achieved by a feedback technique, such as a frequency-locked loop. However, when interference and/or noise also are received over the communication channel, optimum signal reception is not always achieved at precisely such an automatically fine-tuned frequency.
These are particular problems with video signal reception, since differences in the quality of video reception are readily perceived by viewers.